You could use the OpenArgs parameter for the form. When you open the form, use something like:
DoCmd.OpenForm "frmMyForm", acNormal, , , , , myVariable
Then you would have to check for the OpenArgs parameter in the Open event for frmMyForm and handle it as necessary.
Example:
If IsNull(Forms!frmMyForm.OpenArgs) Then
'no parameter passed, do error handling
Else
myNewVariable = Forms!frmMyForm.OpenArgs
End If